1. Field of the Invention
The invention relates to video imaging systems with respect to enhancing image detail and clarity.
2. Description of the Prior Art
Optical devices and systems such as lenses, cameras, televsion cameras, video monitors, television systems, filters, and the like, have a response to spatial frequency denoted as the Modulation Transfer Function (MTF). The MTF is a curve of contrast versus spatial frequency. This is analogous to audio devices having a frequency response of amplitude versus frequency.
As is well known, spatial frequency is the reciprocal of the line spacing of a series of equally spaced lines and is traditionally expressed in cycles per millimeter. In order to obtain the MTF of an optical device, a chart with groups of three black bars on a white background is utilized. The difference between the black and white of the bar set, as measured with a photometer, compared with the black to white ratio of an entirely black target compared with an entirely white target is the contrast at the spatial frequency given by the inverse of the bar group spacing.
All optical devices inherently have a non-ideal MTF response curve because of the finite size of the optical aperture associated therewith. The MTF curve of such optical devices is normally a monotonically decreasing function such as a downwardly sloping diagonal line, or the like, that intersects the spatial frequency axis at a point of frequency less than or equal to the diffraction limit. The diffraction limit is the spatial frequency at which the finite size of the optical aperture renders resolution at higher spatial frequencies impossible. Thus, the diffraction limit is the maximum point at which the MTF curve intersects the spatial frequency axis and is therefore the point on the MTF curve at which the contrast or resolution must diminish to zero. Were the aperture of the optical device infinite, the device would not be limited by the diffraction effects associated with the aperture edges.
In an optical system, the MTF curves of all of the devices in the system are multiplied point by point to provide the system MTF curve. Since each of the optical devices comprising the system has a non-ideal response, the system MTF curve is typically a downwardly sloping function diminishing to zero contrast or resolution at the diffraction limit. The downwardly sloping characteristic of the typical MTF response results in a gradual loss of contrast in the detail of the image as the detail becomes finer and finer until the diffraction limit is attained. An Aperture Correction Filter (ACF) is a device that endeavors to compensate for the diffraction effects of the finite size of the optical aperture of the system. The ACF ideally has an MTF curve that is the inverse of the system MTF curve such that when the ACF is included in the system, the composite MTF curve is substantially flat out to the diffraction limit. The ACF provides a boost in the high spatial frequency contrast to compensate for the decreasing characteristic of the system MTF curve. An ACF is to an optical system what an audio equalizer is to an audio system; viz, compensating for the inadequate frequency response of the system.
The ideal MTF curve for an ACF is the point-by-point reciprocal of the contrast of the MTF curve of the optical system without the ACF. Ideally, the MTF of the system with the ACF is 100% from a spatial frequency of zero out to the diffraction limit of the worst component of the system, which usually is the first element. Further contrast boost beyond the system diffraction limit is ineffective since improvement beyond this spatial frequency is impossible. The system with the ACF as compared to the system without the ACF provides more pronounced detail and therefore produces a visually better, sharper image.
In the prior art, the ACF function has been implemented in television camera systems as an electronic high pass filter on the raster scanned video. This technique boosted the high spatial frequency response of the system tending to compensate for the finite aperture of the camera system. The technique operates and has an effect only on the horizontal axis and is therefore only one dimensional. Such a one dimensional filter does not effect any improvement with respect to fine detail resolution in the vertical dimension. It is extremely difficult to implement a vertical ACF. Prior art aperture correction filters utilizing electronic circuitry tended to be undesirably complex, bulky and expensive. Such prior art filters were difficult to design and it was furthermore difficult to alter the filter characteristics thereof.
The following U.S. patents exemplify the state of the art: 4,080,627; 4,097,897; 4,110,790; 4,160,265; 4,160,276; 4,200,888; 4,328,772; 4,275,417; 4,336,552; 4,402,006; 4,410,912; 4,481,537; 4,524,379; 4,623,923; 4,691,366; 4,709,393; and 4,714,958. Many of the devices of these patents suffer from the defect of providing an affect only in the horizontal dimension. Other devices combine a vertical ACF with a horizontal ACF utilizing undesirably complex, expensive, bulky and slow techniques and apparatus.